The present invention relates to a magnetic recording method utilizing thermo-magnetic printing, magnetic recording media for thermo-magnetic printing, and a magnetic disk recording apparatus in connection with ultra-high density magnetic recording technology.
With the development of information technology, digitalizing in various fields is being rapidly advanced. There arises the need for storing a large amount of digital information data not only in conventional personal computers and servers but also in consumer electronics and audio products. To store the enormous amounts of data, a magnetic disk recording apparatus as the core for a nonvolatile filing system is required to rapidly make its capacity larger more than ever before. Making the capacity of the magnetic disk recording apparatus larger means further increasing a bit density recorded on magnetic recording media, that is, an areal recording density.
A recording method employed in a magnetic disk recording apparatus currently in practical use is generally called longitudinal recording. This is a method for recording information by using a ferromagnetic thin film having a high coercivity in the direction parallel with the surface of a disk substrate as a magnetic recording medium to magnetize the magnetic recording medium along the surface of the substrate. In this case, a magnetization reversal point in which longitudinal magnetizations are opposite to each other at an angle of 180 degrees corresponds to bit 1.
To increase the areal recording density, the bit density in the disk circumferential direction (linear density) and the bit density in the disk radius direction (track density) must be increased at the same time. The track density is limited by a process for pole-shaping of a recording/reproducing head and the positioning accuracy of a mechanism, which are mostly only technical problems. It is considered that increasing the linear density is subject to a principle limit due to the fact that a magnetic recording medium is an aggregate of ferromagnetic particles.
In the longitudinal recording, magnetizations are opposite to each other with respect to a magnetization reversal point, and a large internal magnetic field called a demagnetizing field is produced in the direction reducing the magnetizations around the magnetization reversal point. The demagnetizing field forms in the magnetization reversal point a magnetization transition area having a finite width, that is, an area in which magnetization has not reached sufficient saturation.
When the magnetization transition area is relatively large, the magnetization transition areas interfere with each other as the bit spacing is narrower so that the substantial magnetization reversal position is shifted. The width of the magnetization transition area must be at least smaller than the bit pitch.
To increase the linear density, a magnetic recording medium must overcome the demagnetizing field for magnetization; specifically, the coercivity of the magnetic recording medium must be increased and the thickness of the recording magnetic film must be reduced to suppress the demagnetizing field. The linear density is strongly limited by the structure and the magnetic characteristic of a magnetic recording medium.
In standard longitudinal recording, the ratio of the linear density to the track density is desirably about 5 to 10. When realizing a recording density of 100 gigabits (gigabit=109 bits) per square inch under the condition, the bit pitch in the circumferential direction is about 25 nm. When estimating the necessary characteristic of a magnetic recording medium whose magnetization reversal width is below 25 nm in a simplified model, the film thickness of the magnetic recording medium must be below 15 nm and the coercivity thereof must be above 5 kOe (kilooersteds).
A recording field which can be produced by the magnetic recording device of the longitudinal recording is considered to be only about 9 kOe by assuming 2.4 T (teslas) as the maximum saturation magnetic flux density (hereinafter, Bs) which can be expected to be a usable magnetic pole material. In this case, when the coercivity of a medium recording layer exceeds 5 kOe, it is difficult to secure a recording field which can sufficiently magnetize the magnetic recording medium.
When the thickness of a Co alloy magnetic film is below 15 nm, the substantial volume of the magnetic recording medium crystal particles is small. As compared with a magnetic anisotropy energy (that is, an energy stabilizing magnetization in the fixed direction) of the individual particles, the magnitude of a thermal energy (that is, an energy disturbing magnetization) cannot be ignored.
The thermal fluctuation of magnetization is significant so that there arises the problem of thermal decay of magnetization in which the magnitude of recording magnetization is reduced as time elapses. To suppress the thermal decay of magnetization, the coercivity must be increased or the volume of the crystal particles must be larger.
When the magnetic field of a magnetic head is limited as described above, there is a limit to an allowable coercivity. Increasing the film thickness for making the volume of the crystal particles larger means an increase in magnetization transition area due to increased demagnetizing field, that is, a decrease in possible linear density.
When securing the volume of the crystal particles by the crystal size along the surface of the substrate, the randomness of the magnetization distribution in a magnetic recording medium is increased. The medium noise is increased so that a sufficient S/N ratio cannot be obtained. Principle difficulty is expected to realize longitudinal recording which can satisfy the conditions of robustness for thermal decay of magnetization, low noise and sufficient recording and exceeds 100 gigabits per square inch.
To prevent the principle difficulty, perpendicular recording is proposed. The perpendicular recording is a method for forming the magnetization of a thin-film magnetic recording medium to be perpendicular to the surface of the film, which is different in recording principle from the prior art longitudinal magnetic recording medium. The perpendicular magnetic recording is not affected by a demagnetizing field since adjacent magnetizations are not opposite to each other and are arrayed to be anti-parallel. A magnetization transition area can be expected to be very small, which can easily increase a linear density. From the same reason, requirement to reduce the film thickness of a magnetic recording medium is not stronger than the longitudinal recording. High robustness for thermal decay of magnetization can be secured.
The perpendicular magnetic recording is focused on as a method essentially suitable for high-density magnetic recording. The materials and structures of various magnetic recording media and the construction of a thin-film magnetic head combining them are proposed. The perpendicular recording includes a method for using a medium having a single perpendicular magnetization film and a method for providing, in addition to a perpendicular magnetization film, a flux keeper layer adjacent to its disk substrate side and having a low coercivity and a high saturation magnetic flux density.
When using a two-layered perpendicular magnetic recording medium having the flux keeper layer, there can be considered the following advantages: (1) a demagnetizing field produced in a recording layer can be reduced; and (2) combination with a single-pole type magnetic recording device can produce a large recording field having a distribution steeper than that of a ring head used in the longitudinal recording. This technique is described in Non-Patent Document 1.
As a perpendicular magnetic recording medium of this type, there is studied a medium provided with a perpendicular magnetization film made of a CoCr alloy on the flux keeper layer made of a soft magnetic layer such as a permalloy, an Fe amorphous alloy or a microcrystal alloy. In recent years, there are studied, as recording layers, an artificial lattice film such as (Co/Pd)n or (Co/Pt)n and granular media in which Co magnetic fine particles are dispersed in SiO2. As the keeper layer, there is used a stacked film stabilizing a magnetic domain by using an antiferromagnetic substance or a magnetic multilayer in which ferromagnetic layers are coupled to each other in an antiferromagnetic manner.
As described above, the perpendicular magnetic recording using a magnetic recording medium having a keeper layer is regarded as promising as a technique instead of the current longitudinal magnetic recording. In this method, the recording density is considered to be limited. The most major factor is that the recording field reaches the upper limit in principle so that the magnetic anisotropy energy determining the long-period stability of the magnetic recording medium cannot be increased.
In the perpendicular recording, a recording field larger than the ring head in the longitudinal recording can be expected as described above. In parameter setting assuming that an areal recording density is 1 tera-bit (tera=1012) per square inch, in consideration of the shape of a magnetic field distribution, there is a studied result in which when using a material having the maximum saturation magnetic flux density in a transition metal magnetic substance of about 2.4 T=24 kG (kilogausses), about 16 kOe is the upper limit of a recording field (Non-Patent Document 2). In this case, the anisotropy magnetic field allowable value of the magnetic recording medium is up to about 16 kOe.
In some examples in which the possibility of magnetic recording whose areal recording density is 1 tera-bit per square inch has been studied by simulation, an anisotropy magnetic field of about 20 kOe is necessary and the above is predicted to be insufficient. There is considered a method for realizing a higher recording density by introducing a new element into recording only by a magnetic method. The two candidate methods are mainly considered.
(1) Thermally Assisted Recording (or Hybrid Recording)
The magnetic anisotropy energy of a magnetic recording medium is reduced only when applying a recording field, thereby permitting writing onto a magnetic recording medium having an anisotropymagnetic field larger than the recording field at room temperature. It is possible to use a magnetic recording medium having small crystal particles having a magnetic anisotropy energy larger than that of pure magnetic recording using a recording field of the same intensity, which may achieve a higher recording density.
For this method, some medium structures and head constructions have been proposed (Non-Patent Document 3).
(2) Recording Utilizing Thermo-Magnetic Printing
A magnetization pattern is written onto a specified, layer in a magnetic recording medium by a magnetic field from a magnetic recording device as in normal magnetic recording. The magnetic recording medium is heated to print the magnetization pattern onto another layer. The film thickness of a recording layer is substantially increased to secure long-period stability. The thermal stability may be lowered in a single layer initially written. A magnetic recording medium having small crystal particles can be used, which may achieve a higher recording density.
This method has been studied in the magnetooptical recording technology. In particular, it is considered to be a candidate method for a technique performing direct overwrite not via an erasure process generally conducted in magneto-optical recording.
Patent Document 1 discloses a technique recording onto a medium combining a Co—Cr alloy thin film for magnetic writing with a rare-earth element-transition metal alloy thin film printing its pattern by a method of combining magnetic recording with light exposure. Patent Document 2 discloses a technique printing a magnetization pattern from an “assist layer” increasing the coercivity by heating to a “recording layer” decreasing the coercivity by heating.
[Patent Document 1]
Japanese Patent Application Laid-Open No. Sho 63-276731
[Patent Document 2]
Japanese Patent Application Laid-Open No. Hei 2-189751
[Non-Patent Document 1]
IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 657-662, Perpendicular magnetic recording—Evolution and future ′
[Non-Patent Document 2]
IEEE Transactions on Magnetics, Vol. MAG-39, No. 4, July 2003, pp. 1955-1960, Recording field analysis of narrow-track SPT head with side shields, tapered main pole, and return path for 1 Tb/in2′
[Non-Patent Document 3]
Journal of Applied Physics, Vol. 87, No. 9, May 2003, pp. 5398-5403, ‘Disk recording beyond 100 Gb/in2: Hybrid recording?’